1. Field of Invention
The present invention relates to an analog to digital converting device, and more particularly to an analog to digital converting device that has a plurality of photo relays to form multiple input channels, since the impedance of the photo relay is extremely high, the isolation effect between two input channels is improved and possible damage to the channels caused by high voltage is avoided.
2. Related Art
In an operating system, analog signals often need to be converted into digital signals firstly, whereby the operating system can analyze and process the digital signals. Thus an analog to digital converter acts as an important element. Conventionally, the A/D converter uses a multiplex switch to select one of multiple analog signal, and then the A/D converter converts the selected analog signal into a digital form.
The multiplex switch is an essential element acting as a signal switching device. With reference to FIG. 3, a conventional multiplex switch has several input channels (CH1-CH8), and each input channel (CH1-CH8) has a positive and a negative input terminal. Both the positive and the negative input terminal are respectively connected to two common output terminals (A, B) through an analog switch (30). A decoder (31) is provided to control the multiple analog switches (30) to turn on/off. When the decoder (31) triggers the analog switches (30) in the channel 1 (CH1) to simultaneously turn on, the signals inputted from the channel 1 (CH1) are able to conduct to the common output signals (A, B), meanwhile the remaining channels (CH2xe2x89xa7CH8) are all cut off.
However, a poor isolation problem exists in the conventional multiplex switch. With reference to FIGS. 4A and 4B, if the channel 1 (CH1) is activated and the remaining channels (only channel 2 is shown) are cut off, once an undesired interference voltage (32) is inducted across channel 1 (CH1) and channel 2 (CH2), since the impedance of the analog switches (30) in the channel 1 (CH1) and channel 2 (CH2) are not high enough to bear the interference voltage (32), both the channel 1 (CH1) and channel 2 (CH2) are easy to be broken by the high potential of the interference voltage (32). The interference voltage (32) may occur because of many factors, such as static interference among the channels, lightning or electromagnetic interference.
To overcome the shortcomings, the present invention tends to provide an A/D converting device so as to mitigate and obviate the aforementioned problems.
An objective of the invention is to provide an analog to digital converting device that has a photo-coupled switching device, the switch has multiple input channels and each input channel has two photo relays acting as switching elements, and because of the high impedance of each photo relay, the isolation between two adjacent input channels is enhanced whereby possible damage from the interference voltage is avoided.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.